The present invention relates generally to digital data processing and transmission techniques, and particularly to systems and methods for transmitting digital video images within limited bandwidths.
Presently, there are many systems in which video images are desired to be transmitted by means of electrical or electromagnetic signals. Examples of such transmitted video images are television, electronic photographs, facsimiles, video mapping, etc. Particularly when color video images are to be transmitted, the transmission system generally requires considerable bandwidth for the signal. While large bandwidth systems, such as a typical television transmission system, can often adequately transmit a video signal, a considerable amount of bandwidth is required, and the transmitting and receiving equipment must be relatively complex and expensive to process large bandwidth signals. Additionally, in military applications, large bandwidth transmissions are more readily detected and, therefore, more susceptible to jamming than more narrow band signals.
In many video image systems, a video image to be transmitted is divided into an array of picture elements or pixels. Each pixel represents the video image at one small point of the pixel array. In some systems, a pixel may be represented by a single digital bit, either a zero or a one, indicating either the presence or absence of white in the portion of the image represented by the pixel. In more sophisticated systems, each pixel is represented by plural digital bits which permit each pixel to have more than binary values of zero and one. For example, if four bits are used to represent a pixel then the pixel may have up to sixteen different values, generally ranging from white to black. Each increment from one to sixteen in the binary digits often represents a darker or lighter shade of gray transitioning from white to black. Finally, in color systems, each pixel may be represented by three sets of plural digital bits, each of the plural digital bits of a set representing the amount of one of the primary colors (red, blue or green) present at the portion of the visual image represented by the pixel. Such a scheme is often utilized in digitizing television images.
Because standard broadcast color television utilizes a video image which is 768 pixels wide and 512 pixels high, each screen image of a television contains approximately 393,000 pixels or approximately 9.4 million bits of digital data for a single screen image. While such large amounts of data can readily be sent by large bandwidth transmission and receiving equipment, it is often desired to send such digital video images by less expensive and more readily available low speed equipment such as HF (high frequency) radio and telephone voice lines. Such low speed devices typically operate at anywhere from 1200 to 9600 bits per second. If a single digital television image is sent via such a low speed transmission link, for example, a 2400 bps link, approximately 65 minutes would be needed to send a single image. In a tactical military situation, a transmission of such duration is usually desired to be avoided as such a long duration transmission could readily be detected and jammed. Moreover, such a long transmission time for each image would permit only a single image to be sent per hour.
Many of the known video image transmission systems transmit an entire screen of data. Often, however, only certain portions of the screen are of interest to the recipient of a signal and some portions of the image may be of more importance than others. In most known video signal transmission systems, no provision is made for transmission of only a selective portion of the video image or for transmission of different portions of the image at different resolutions.
While visual images in the form of television signals may be transmitted quite accurately by VHF equipment, VHF transmissions are generally limited to the line of sight between the transmitter and the receiver. Consequently, it is often desired to send television and other video image signals at other than VHF frequencies. For example, the use of signal carriers in the high frequency ("HF") range, i.e., 3,000 to 30,000 KHz, often permits worldwide communication between transmitter and receiver as the HF signal can be bounced or reflected off the ionosphere and back to earth. Conventional HF systems do not have sufficient bandwidth to send real time television signals, however.
It is accordingly an object of the present invention to provide a novel video image system and method whereby digital video images may be transmitted worldwide and/or through relatively low bandwidth transmission media.
It is a further object of the present invention to provide a novel system and method for video image transmission whereby selected portions of a video image may be transmitted to a receiver.
It is yet another object of the present invention to provide a novel video image system and method in which the resolution of the image to be transmitted may be controlled by the operator of the video system in accordance with the operator's interest in the contents of the video image.
It is still a further object of the present invention to provide a novel system and method of video image transmission whereby the portion of the image of most interest to the user is transmitted first.
It is still another object of the present invention to provide a novel video image system and method in which the operator of a video transmission system is made aware of the progress of the transmission of video image data.
It is still yet another object of the present invention to provide a novel video image system and method in which the time during which transmission takes place is limited to relatively small bursts and in which successive transmissions may be made at different transmission frequencies.
These and many other objects and advantages of the present invention will be apparent from the claims and from the detailed description of the preferred embodiments when read in conjunction with the appended drawings.